Process vessels are tanks or other containers used in the semiconductor industry as part of the device fabrication process. Process vessels are the containers in which fabrication processes such as the etching, cleaning, stripping, and rinsing of semiconductor wafers are carried out. One type of process vessel, termed a rinse tank, is used during the cleaning of the substrates or wafers on which devices are to be fabricated. The cleaning process removes particles and contaminants acquired during manufacture of the wafers. This is necessary because the presence of contaminants or residues on a substrate can adversely affect the electrical characteristics and yield of the fabricated devices.
During a cleaning process, the substrates or wafers are typically placed in a slotted cassette (sometimes termed a "boat") which is placed in a tank (process vessel) containing a cleaning agent, typically an acid bath. The acid bath is commonly formed from a mixture of an oxidant species and an acid. After immersion in the acid bath, the cassette is transferred to a rinse tank in which the treated wafers are rinsed in ultrapure water. The rinsing step is designed to quench the wafers, so that the acid solution does not continue to act on the wafers, and to dilute and remove the chemical residues from the wafer surface.
Continual efforts are made in the semiconductor industry to improve rinse tank designs in order to reduce rinse water consumption and increase wafer throughput. This is especially important as the number of device fabrication steps and wafer size increase. These efforts generally involve making measurements to determine how quickly and completely the contaminants and oxidant solution are removed from the surface of the wafers during the rinse process. Methods currently employed to improve rinse tank and rinse process efficiency include minimizing the tank volume, reducing the wetted surface area by use of a reduced cassette, utilizing dilute chemistries, and increasing the temperature of the rinse water.
In addition, rinse tank structure designs can be optimized through the use of computational fluid dynamics, followed by experimental verification. To date, most published data used to characterize and model the rinse process within a rinse tank (or another fabrication process being carried out in a different process vessel) has been acquired by means of a wall mounted conductivity probe. While this provides useful information, there are many drawbacks to wall mounted probes: 1) The probe does not provide a direct measurement of the conductivity (and by inference, ion concentration) in the wafer gap between two wafers; 2) Probe performance is affected by changes in tank configuration and flow field; and 3) The probe response to different carry over chemistries does not correlate quantitatively to the amount of carry over on the wafer during rinsing. Because the placement of the probe on a tank wall interferes with the normal fluid flow within the rinse tank, the measurements obtained in this manner contain an inherent source of error as they are based on a disturbed tank flow and are measured at a location way from the surface of interest.
Other process vessels, such as those used to perform etching or photoresist stripping stages of the fabrication process are also candidates for investigation and design improvements. In order to improve such process vessels and the related fabrication processes, it is desirable to have a means of collecting accurate data on the impact of changes of the relevant vessel variables on the operation of different process vessel designs. This permits the identification of design changes which improve the efficiency of the processes carried out in the vessels.
What is desired is an alternative type of conductivity cell or probe which overcomes the disadvantages of wall probes currently used for characterizing the fluid flow within a process vessel and investigating the efficiency of fabrication processes conducted within the vessel.